The present invention relates to active suspension systems for vehicles, and in particular to such systems which include active roll control.
It is known from U.S. Pat. No. 5,487,006 to provide a vehicle suspension system in which a single lateral accelerometer is used to give an indication of the nature of the surface over which the vehicle is travelling. However this system suffers from the disadvantage that it cannot clearly distinguish between lateral acceleration of the whole vehicle, and lateral acceleration caused by body roll produced by a rough road surface.
The present invention provides apparatus for detecting the roughness of a surface over which a vehicle is travelling, the apparatus comprising roll detection means for measuring roll movements of the vehicle, filtering means for detecting high frequency roll movements, and control means arranged to measure the said roughness by measuring the level of said high frequency roll movements.
Preferably the roll detection means comprises a pair of accelerometers located on the sprung part of the vehicle at different distances from the roll axis of the vehicle.
Preferably the accelerometers are vertically spaced and each accelerometer is arranged to detect acceleration in a direction which is lateral to the vehicle.
Preferably the control means is arranged to monitor the measured acceleration from each of the accelerometers, to produce a roll signal dependent on the difference between the two measured accelerations said roll signal being indicative of instantaneous vehicle roll.
Preferably said difference is an integral over time of the instantaneous difference between the measured accelerations, or the difference between respective integrals over time of the two measured accelerations.
The present invention further provides a vehicle suspension system including apparatus according to the invention wherein the control means is arranged to control the roll stiffness of the vehicle, and is arranged to reduce the roll stiffness of the vehicle in response to the detection of increased surface roughness.
Preferably the system further comprises vehicle speed detection means wherein the control means is arranged to increase the roll stiffness in response to increasing vehicle speed.
The present invention also provides a vehicle suspension system comprising roll control means for controlling the roll stiffness of the vehicle, roughness measuring means for measuring the roughness of the surface over which the vehicle is travelling, vehicle speed measuring means for measuring the speed of the vehicle wherein the roll control means is arranged to increase the roll stiffness if the vehicle speed increases but to decrease the roll stiffness of the roughness of the surface over which the vehicle is travelling increases.